Abrasion tester and method of testing abrasion resistance of flexible sheet material

ABSTRACT

AN ABRASION TESTER HAVING A MOVING ABRASIVE SURFACE AND A COUNTER-WEIGHTED SAMPLE HOLDER FOR HOLDING A FOLDED CORNER OF FLEXIBLE SHEET MATERIAL IN CONTACT WITH THE MOVING ABRASIVE SURFACE AND A METHOD OF DETERMING THE ABRASION RESISTANCE OF A FOLDED CORNER OF FLEXIBLE SHEET MATERIAL WHEREIN THE CORNER IS SUBJECTED TO A PREDETERMINED AMOUNT OF ABRASION AND THEN TESTED FOR PREMEABILITY TO AN INDICATING FLUID TO WHICH THE SHEET MATERIAL IS NORMALLY IMPERMEABLE.

Jan. 12, 1971 7 H 3,554,007

. ABRASION TESTER AND METHOD OF TESTING ABRASION RESISTANCE OF FLEXIBLE SHEET MATERIAL 2 Sheets-Sheet 1 Filed Aug.- 7; .1969

Filed Aug. v. 1969 Jam-12,1971 I I MU 554700? ABRASION TESTER AND METHOD OF TESTING ABRASION RESISTANCE OF FLEXIBLE SHEET MATERIAL 2 Sheets-Sheet 2 a4 536 3e 4e 2 as 2' 2: //O as 8 a (539 I I I I4 United States Patent 3,554,007 ABRASION TESTER AND METHOD OF TESTING ABRASION RESISTANCE 0F FLEXIBLE SHEET MATERIAL Kwoh H. Hu, Wayland, Mass., assignor to the United States of America as represented by the Secretary of the Army Filed Aug. 7, 1969, Ser. No. 848,284 Int. Cl. G01n 3/56 US. Cl. 73--7 Claims ABSTRACT OF THE DISCLOSURE An abrasion tester having a moving abrasive surface and a counter-weighted sample holder for holding a folded corner of flexible sheet material in contact with the moving abrasive surface and a method of determining the abrasion resistance of a folded corner of flexible sheet material wherein the corner is subjected to a predeter mined amount of abrasion and then tested for permeability to an indicating fluid to which the sheet material is normally impermeable.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to an abrasion tester of the type comprising a rotary platform driven by a motor and having an abrasion surface thereon and comprising means for supporting a triangularly shaped, twice-folded, flexible sheet material so that a folded corner thereof bears upon the abrasive surface under a predetermined force during rotation of the platform. The invention also relates to a method of testing the abrasion resistance of a folded corner of a flexible sheet material which is normally impermeable to a predetermined fluid.

When a flat sheet of flexible material is used to produce a flexible package or container, it is inevitable that corners and folds are formed in the flexible material. Sustained or intermittent vibration and rubbing of these corners and folds during transportation of such packages frequently results in breaks in the flexible material which may endanger the contents of the packages to contamination and spoilage, particularly in the case of a food product. It is, therefore, important to know the relative abrasion resistances of various type of flexible sheet materials for packaging supplies, such as foods, which must be shipped great distances and stored at relatively high temperatures for long periods of time prior to use of the contents of the packages. It is particularly important to know the relative abrasion resistances of corners formed by folding such flexible sheet materials that are under consideration for use by the Armed Forces, which may require packages capable of withstanding the abrasion resulting from shipping supplies long distances and at times over rough terrain or under other conditions which may cause such packages to be severely abraded.

' It is, accordingly, an object of the present invention to provide an abrasion tester for testing the susceptibility of a folded corner of a flexible sheet material to damage from abrasion.

A further object of the invention is to provide a method of testing the abrasion resistance of a folded corner of a flexible sheet material.

Other objects and advantages will appear from the description of the invention hereinafter and the accompanying drawings wherein:

FIG. 1 is a side view in elevation of the abrasion tester of the invention;

FIG. 2 is a top plan view of the abrasion tester of the invention;

FIG. 3 is a front view in elevation of the abrasion tester of the invention;

FIG. 4 is a plan view of a square sheet of flexible material prior to being folded for the purpose of testing in the abrasion tester of FIGS. 1-3;

FIG. 5 is a plan view of the sheet of flexible material of FIG. 4 folded along a diagonal first fold line, forming a first triangle with two thicknesses of the material superimposed; and

FIG. 6 is a plan view of the sheet of flexible material of FIGS. 4 and 5 folded a second time along a second fold line perpendicular to the first fold line from about the midpoint thereof to the opposed corner of the triangle of FIG. 5, thus forming a second triangle half the size of the first triangle with four thicknesses of the flexible material superimposed.

As seen in FIGS. 1-3, the abrasion tester 10 comprises a base 11, which may be a hollow metal casting, to the bottom of which resilient cushions 12 are attached to serve as vibration absorbing supports for the abrasion tester. Mounted within the base 11 is an electric motor (not shown) having a vertically oriented shaft 13 on which is mounted a circular rotary platform 14. An abrasive surface 15 is clamped on the upper face of platform 14 by means of circularly shaped peripheral clamp 15 and disc-shaped inner clamp 17. Clamp 17 is held tightly clamped against abrasive surface 15 by means of thumbscrew clamp 18 which screws onto the upper end of shaft 13. Rotary platform 14 rotates in a horizontal plane causing rotation of abrasive surface 15 also in a horizontal plane. The number of revolutions of the rotary platform is registered on revolution counter 19, which can be reset for subsequent tests by means of reset knob 20. Switch 21 controls the operation of the electric motor.

Mounted on or integrally cast with base 11 are upright supports 22 and 23 which are adapted to support portions of the abrasion tester which are particularly novel to this type of tester. Shaft 24 passes through opening 25 in upright support 22 and coaxial opening 26 in upright support 23 and is held stationary by means of setscrews (not shown). A retaining bracket 27 which is bifurcated at both its lower and upper portions, having lower arms 28 and 29 and upper arms 30 and 31, is

mounted on shaft 24 by means of openings 32 and 33 in Retainer assembly 36 comprises a lever arm 37, fulcrum pivot pin holder 38, and sample retainer arms 39 and 40. Lever arm 37 is threaded so as to cooperate with three counterweights 41 which are internally threaded to cooperate with the threads of lever arm 37 whereby counterweights 41 may be adjustably located along lever arm 37 so as to permit the application of more or less force at the free ends of sample retainer arms 39 and 40 and to any test sample retained therebetween. Fulcrum pivot pin 42 is provided with a relatively sharp point at both ends thereof and passes through fulcrum pivot pin holder 38 to which it is aflixed by brazing or otherwise. Bearing screws 43 and 44, which are externally threaded so as to cooperate with internally threaded openings passing through arms 30 and 31, repectively, have bearing surfaces machined or otherwise formed in the ends thereof which are opposed to each other across the space between arms 30 and 31 in which fulcrum pivot pin holder 38 and fulcrum pivot pin 42 are located so as to provide cooperation between the pointed ends of fulcrum pivot pin 42 and the bearing surfaces in the ends of bearing screws 43 and 44. The bearing surfaces may be adjustably separated from each other by turning bearing screws 43 and 44 and tightening jam nuts 45, thus providing for free pivotal motion of pivot pin 42 with respect to the bearing surfaces in bearing screws 43 and 44.

Sample retainer arms 39 and 40 are generally flat, oblong pieces of metal, each being outwardly bowed over a short portion of its length and having a bolt 46 passing through both bowed portions of sample retainer arms 39 and 40 which with knurled nut 47 is adapted to clamp sample 48 between the free ends of sample retainer arms 39 and 40 for the purpose of testing its abrasion resistance. The ends of sample retainer arms 39 and 40 are brazed or otherwise attached to the fulcrum pivot pin holder as best seen in FIGS. 2 and 3.

The flexible material sample 48 is prepared by first cutting or otherwise preparing a generally square sheet 49 of flexible material as shown in FIG. 4. For testing in a corner abrasion tester constructed to approximately double the scale of the tester shown in FIGS. 1 3, it has been found advantageous to employ a sheet of flexible material approximately 3 x 3 inches square. This square sheet 49 is folded diagonally along a first fold line 50 to form a triangularly shaped intermediate sample 51 having two superimposed thicknesses of the flexible material triangular in shape. The triangularly shaped intermediate sample 51 is then folded again along a second fold line 52 which is perpendicular to the first fold line 50 from about the midpoint thereof to apex 53 of the triangular intermediate sample 51 thus producing the triangularly shaped flexible material sample 48 comprising four superimposed thicknesses of the flexible material and having a closed corner 54.

In carrying out a test of the corner abrasion resistance of a sample of flexible material 48 prepared as described above, the sample 48 is inserted between the free ends of sample retainer arms 39 and 40 with these arms somewhat separated by unscrewing knurled nut 47, the sample 48 being inserted so that closed corner 54 will be oriented downwardly, as shown in FIGS. 1 and 3, so that the closed corner bears upon abrasive surface 15, Knurled nut 47 is tightened to hold sample 48 between sample retainer arms 39 and 40. Counterweights 41 are adjusted to allow the closed corner of the flexible material sample to bear against abrasive surface 15 with the desired force, Revolution counter 19 is set at zero. The test is begun by turning on the motor by means of switch 21 to start the revolving of rotary platform 14 carrying abrasive surface 15. Closed corner 54 of flexible material sample 48 is thereby subjected to abrasion by the rubbing action of abrasive surface 15 thereagainst as the rotary platform continues to revolve. In practice, a sample is subjected to a preselected number of revolutions of the rotary platform and the abrasive surface. The tester is then stopped. The sample Cir is removed from between retainer arms 39 and 40 and tested with a predetermined fluid to which the sample material is normally impermeable to determine whether the closed corner 54 has been abraded through. This test is accomplished by partially opening the flexible material sample 48 along the side of the triangle opposite the closed corner 54 introducing a small quantiy of an appropriate fluid, such as a 0.05% aqueous solution of Crystal Violet containing 1000 parts per million of Tergitol Nonionic TMN surfactant, manufactured by Union Carbide Corporation, into the interior of the triangular flexible material sample 48, and bringing a piece of filter paper or other absorbent material into contact with closed corner 54. If the closed corner has been abraded through, the colored liquid will be absorbed by the filter paper or other absorbent material and can be easily detected visually. If the closed corner has not been abraded through, none of the colored liquid passes through closed corner 54 since the colored liquid is preselected to be of a type which will not permeate through the flexible material and, therefore, the filter paper or other absorbent material remains free of the color of the colored liquid. The above-described test is repeated a number of times at each selected number of revolutions of the rotary platform and the abrasive surface to obtain statistically significant values for the abrasion resistance of the flexible material being tested.

It is necessary, of course, to vary the abrasive surface to fit the flexible materials being compared as to abrasion resistance. It is also necessary to vary the force applied to the flexible material sample. In general, the more abrasion resistant the flexible material, the greater the force needed to be applied to the sample and the more abrasive the abrasive surface needed to result in abrasion through the closed corner 54 within a reasonable number of revolutions of rotary platform 14 and abrasive surface 15.

The following example illustrates the use of the abovedescribed corner abrasion tester in testing and comparing the abrasion resistances of several laminated flexible materials which were under consideration for possible use for flexible packaging of food products.

EXAMPLE The following laminated flexible sheet materials were tested on the above-described corner abrasion tester to determine their relative resistances to corner abrasion:

(a) 0.5 mil polyester (polyethylene terephthalate)/ 0.35 mil aluminum foil/3.0 mil high density polyethylene (manufactured by Dow Chemical Company).

(b) 2.0 mil polyester (polyethylene terephthalate)/ 0.35 mil aluminum foil/2.0 mil vinyl (manufactured by Anaconda Aluminum Company).

(c) 0.5 mil polyester (polyethylene terephthalate)/ 0.35 mil aluminum foil/ 3.0 mil modified polyolefin (manufactured by Continental Can Company).

(d) 0.5 mil polyester (polyethylene terephthalate)/ 2.0 mil low density polyethylene (manufactured by Dow Chemical Company as Dow No. 1367).

Samples of the sheet materials described above were prepared as described above, using 3 x 3 inch squares and folding the materials twice, as described above, so that the polyester layer was on the outside of each triangular sample in each case.

A counterweight of 23.5 grams was located on lever arm 37 a distance of 3.4 centimeters from the fulcrum formed by pivot pin 42. The abrasive surface 15 was emery polishing paper of grit No. 0. The sample was held so that closed corner 54 rested on the abrasive surface with each of the sides of the triangle forming the closed corner oriented at approximately 45 degrees with respect to the abrasive surface. The rotary platform 14 was mated at 36 revolutions per minute.

Ten samples of each of the above flexible materials were tested at each of three predetermined numbers of revolutions of the rotary platform. At the conclusion of each series of revolutions of each sample, the sample was removed from the tester and tested with crystal violet solution, as described above, to ascertain whether the closed corner had been abraded through. Table 1 gives the results of lthe observations for'the four laminated flexible mater1a s:

TABLE 1.-SAMPLE MATERIALS a o c No No No Break Break Break Break Break Break No Break Break No. of revolutions:

It is apparent from the above results that the four laminated flexible sheet materials have corner abrasion resistances decreasing in magnitude in the following order: a, b, c, d.

While the method of the invention has been described above in terms of use of an aqueous solution of crystal violet containing a small quantity of surfactant as the predetermined fluid to which the flexible material of the sample is impermeable, but which passes through any hole abraded through the flexiblematerial at the closed corner subjected to abrasion in the corner abrasion tester and is absorbed by filter paper or other absorbent material, thus indicating when the closed corner has become abraded through, it is to be understood that numerous other colored fluids may be employed for this purpose. The primary requisites are that the predetermined fluid be incapable of permeating or penetrating through the flexible material until a definite hole has been abraded through the latter and that the predetermined fluid be capable of imparting a color to an absorbent material, such as filter paper,

when absorbed therein so that even the slightest penetration of the predetermined fluid through a hole in the flexible material will be easily detectable. Any ink or dye solution which will not permeate or dissolve the flexible material to be tested will operate satisfactorily to indicate when the flexible material has become abraded through. Acid or base indicator papers may be used instead of filter paper and a solution of an acid or base may be employed as the predetermined fluid to be introduced into the sample after abrasion thereof.

The method of the invention has been described above with respect to the application thereof to the testing of laminated flexible materials which have been proposed for flexible packaging of food and other products. However, it is to be understood that the method may be applied to single layers (non-laminated) of many different types of flexible packing materials or to other laminates than those described above. For example, non-laminated polyethylene terephthalate sheets may be tested in accordance with this method. Also, regenerated cellulose sheet material, cellulose ester sheet material, such as cellulose acetate, cellulose triacetate, and other cellulose ester materials may be similarly tested. The method may also be employed in testing the corner abrasion resistance of numerous other flexible sheet materials prepared from synthetic polymeric materials, such as polytetrafluoroethylene, poly (monochlorotrifluoroethylene), polystyrene, high density polyethylene, polyvinyl chloride, polyvinylidene chloride, and many copolymers prepared comprising monomers from which the above polymers are prepared. However, the invention has been found to be particularly advantageous in evaluating and comparing laminated sheet materials which have been found to be effective barriers against moisture and oxygen and which can be readily heat sealed to form strong closures in flexible packages.

platform driven by a motor, said rotary platform having an abrasive surface thereon, and means cooperating with said rotary platform for counting the number of revolutions made by said rotary platform, the improvement comprising means for supporting a triangularly shaped twice folded sample of flexible sheet material so that the tip of a folded corner of said sample representing a point at about the center of said sheet material when unfolded bears upon the abrasive surface of said rotary platform during rotation thereof, said supporting means being counterbalanced through a fulcrum by counter weights adjustably mounted on an arm of said supporting means on the opposite side of said fulcrum from said sample of sheet material.

2. An abrasion tester according to claim 1, wherein said supporting means comprises a bifurcated clamp comprising two arms between which said twice folded sample of sheet material is held substantially perpendicular to said rotary platform and with the tip of said folded corner thereof bearing on said abrasive surface.

3. Method of testing the abrasion resistance of a folded corner of flexible sheet material, said corner being formed by double folding of said sheet material so that four thicknesses of said sheet material are superimposed, comprising the steps of:

(a) folding a sheet of said sheet material along a first fold line to superimpose two thicknesses of said sheet material;

(b) folding said once folded sheet material along a second fold line perpendicular to said first fold line to superimpose four thicknesses of said sheet material and form a folded corner of said sheet material which is closed at the tip thereof;

(c) abrading the material of the external surface of said folded corner on the tip thereof by rubbing said tip against an abrasive surface over a predetermined distance and under a predetermined pressure causing said tip to bear against said abrasive surface; and

(d) detecting the permeability of said abraded tip of said sheet material to a fluid to which said sheet material is normally impermeable.

4. Method of testing the abrasion resistance of a folded corner of flexible sheet material in accordance with claim 3, wherein the permeability of said abraded tip is detected by partially opening said folded corner, placing a quantity of fluid to which said sheet is normally impermeable inside said folded corner and bringing a piece of filter paper into contact with the outside of said abraded tip, said filter paper being absorbent to said fluid and capable of indicating the presence of said fluid thereon by a change in the color of said paper.

5. Method according to claim 3, wherein said sheet is square, said first fold line is along a diagonal of said square, and said folded corner is at one apex of a triangle formed from four superimposed thicknesses of said sheet material.

(References on following page) 8 References Cited FOREIGN PATENTS UNITED STATES PATENTS 199,477 7/1967 U.S.S.R. 73--7 4/1952 Clapham 73-7 12/1957 Press S. CLEMENT SWISHER, Prlmary Exammer 12/ 1959 Norton 116114.19 5 D. M. YASICH, Assistant Examiner 

